In general, a MH will cover a 2'x2' area. A parabolic reflector will change this equation drastically. A good parabolic relector will add an additional 30% or so more light into the tank. It will also spread the light over a bigger foot print. My tank is 6' long, with the MH mounted 18" from the ends. This puts 18" to either side of the bulbs that would need light, with 36" between them. At the bottom of the tank, the light cast by the MH overlap in the middle. I used to have a picture that actually showed this, but website is gone. The bigger problem is with the center brace. I don't really mind the lower light area under the brace, not all of my corals could withstand the MH + VHO lighting, and do much better under the glass brace getting only direct VHO through the center brace. I just consider it different light zones and landscape accordingly. My concern is more with algea growth, I assume the MH will be PLENTY of light, but will algea be a concern? Will the VHO be ENOUGH light?